Since the establishment of Aedes albopictus (the tiger mosquito) in southeastern France in 2004, this mosquito has colonized the entire Mediterranean coastline and has gradually spread into the Rhône Valley and toward Aquitaine. Its northward expansion is more recent and occurs in the form of small, scattered colonization hotspots, affecting limited geographic areas. Its seasonal activity in southern France lasts approximately 6 months.

Certain arboviruses, such as dengue and chikungunya, have the ability to replicate in the salivary glands of Ae. albopictus. The mosquito acquires these viruses through a bite, during the viremic period, of a person who was infected during a stay in an endemic area (imported case). The geographic origin of imported arbovirus cases is predominantly in the overseas departments, particularly the Antilles. The risk of importation is proportional to the scale of epidemics in these endemic areas. Furthermore, not all viruses circulating in areas colonized by Aedes aegypti are well-adapted for transmission by Ae. albopictus, as was the case with the chikungunya virus genotype responsible for the 2014 epidemic in the Caribbean (reducing the risk of transmission in mainland France). To date, vector-borne transmission of Zika has been observed only in areas colonized by Ae. aegypti (possibly in association with Ae. albopictus) and not exclusively by Ae. albopictus (i.e., outside mainland France).

All these factors (size of the population exposed to the mosquito, annual variability in the risk of importation, and viral characteristics) make it difficult to assess the risk of local transmission, which varies greatly by region and year.

To reduce the risk of local transmission of these arboviruses, the DGS’s national plan recommends, without distinction across all departments colonized by Ae. albopictus, early reporting of suspected imported cases prior to diagnostic confirmation, with the aim of enabling immediate intervention by mosquito control services.

In the Pays de la Loire region, two municipalities are colonized by Ae. albopictus: Fontenay-le-Comte (85) since 2015 and the southeastern area of Trélazé (49) since 2017. The classification principles of the national plan have led to the Vendée and Maine-et-Loire departments being classified as Ae. albopictus establishment zones (Level 1), even though the proportion of the population actually exposed is very low (estimated at less than 2% in Vendée and 0.3% in Maine-et-Loire).

The situation in the Pays de la Loire region is therefore not comparable to that of southern France. An approach tailored to the epidemiological, entomological, and climatic context should prioritize the following actions:

• Promote the diagnosis of arboviral diseases through early testing (PCR and NS1 antigen for dengue). Consider only biologically confirmed cases. Testing for chikungunya infection must be systematically considered in suspected cases returning from Africa and Asia.

• Improve the completeness of mandatory reporting of arboviral diseases, which, as in most regions, is of poor quality due to poorly defined objectives.

• Prioritize mosquito control efforts for confirmed cases who have stayed in one of the two municipalities colonized by Ae. albopictus.

• Monitor the geographic spread of mosquito colonization through reports from the public on dedicated websites, and supplement this with high-quality entomological surveillance.

• Train the public to control larval breeding sites in colonized areas.

• Inform travelers of risks specific to the epidemiological situation in the countries they are visiting. In 2018, the greatest risk of importation is linked to the ongoing dengue epidemic in Réunion.